Printing devices are known which have spacer elements that can compensate for surface unevenness on printable material during the transfer of a print image. In conventional printing methods, a print image is applied to a printable material by rolling cylinders or drums which carry the print image and guide the printable material. The print image is produced on a printing plate, and, if necessary, it is passed along to the printable material by means of a transfer cylinder.
In direct printing processes, such as high-relief printing using a flexible printing process i.e., a printing plate equipped with elevated printing surfaces, the print image is applied directly to the printable material.
In offset printing, which is an indirect printing process, the print image is transferred from a printing plate to a transfer cylinder, which is called the blanket cylinder. From there, the print image is transferred to the printable material. Normally, a flexible layer is applied to the blanket cylinder. In standard offset printing, a so-called printing blanket or rubber cloth is used. This printing blanket is dimensionally stable in its expansion plane but deformable in the transverse direction. It also can be compressible to a certain extent in the direction of its thickness. In this regard, it is known to provide compressible layers within a printing blanket.
Rubber cloths or printing blankets used in offset printing are required in the offset printing process for the transfer of the print image, and in particular, to compensate for surface unevenness in the printable material. Since the printing plates for offset printing usually consist of thin sheet metal or foils, they cannot adapt sufficiently to the surface of the printable material.
For satisfactory printing quality, a printing blanket must be pressed against the surface of the printable material with a relatively high contact pressure by using the blanket cylinder against the counter-pressure cylinder. This cannot be avoided even by the use of so-called compressible printing blankets, since they merely have greater deformability than the standard printing blankets, but yet still require a relatively large contact pressure.
Thus, when printing on uneven printing materials, there are limits to the conventional offset printing process. First, a printing blanket can only be deformed to a limited extent, and sufficient pressing of the printing blanket against the printable material to maintain an acceptable print quality is still required. The printable material has an equally limited load resistance.
Thus, for example, when printing on corrugated cardboard, a particular problem arises by reason of the internal structure of the corrugated carton. Since a corrugated connecting layer is provided between two covering layers in order to reduce the weight, a corrugated cardboard carton is not a homogeneous material. The cover layers used on the upper and lower sides of this type of corrugated cardboard are connected by means of a corrugated intermediate layer and only at certain points. When the surface of corrugated cardboard is stressed, irregular conditions appear across the surface. In particular, each cover layer is flexible in the regions between the connecting points with the corrugated intermediate layer, and thus these regions yield when stressed. The stresses occurring on the board during a printing process can adversely affect the uniformity in printing.
Therefore, it is now known to apply a flexible and compressible substrate beneath the printing plate in the flexible printing method. In this regard, it is assumed that the printing plate is itself not compressible, that is, it is rigid and the substrate permits movement of the printing plate with respect to the printing plate cylinder. As substrate, for example, a foam material is used which is located on a substrate that can withstand tensile load. By means of the load-resistant substrate, the flexible substrate can be tensioned simultaneously with the printing plate on the printing plate cylinder. Length expansion is not expected in this case. In this manner, the printing plate is permitted to follow the surface unevenness on the printable material, at least to a limited extent. The necessary deformation of the printing plate, however, results in a deterioration in print quality. This also results from the usually very inhomogeneous distribution of printed and non-printed regions on a printing plate. In addition, the registration of printing plates for the various print colors is very difficult.
In offset printing this type of procedure is not known or used. Of course, stretching the so-called substrate sheet on the printing blanket cylinder underneath the printing plates or printing blankets is a known procedure. But they only help with the alignment, and the perimeter of the printing blanket cylinder stretched with the printing blanket, or the perimeter of the forming cylinder tensioned with the printing plate must be adjusted to the conditions required in a printing process.
Furthermore, another method is used which is not comparable to the method of using the substrate for a printing plate. This method uses compressible printing blankets. A compressible printing blanket permits deformations due to thickness reduction only under comparatively high pressure. The deformations in these printing blankets required for adaptation to the surface structure of the corrugated cardboard in a printing process would necessarily result in destruction of the corrugated cardboard before the printing blanket could sufficiently deform. Therefore, no method has been known for use in offset printing that enables a uniform print distribution in the processing of corrugated cardboard in a single printing process.